The Nevada Rural Ozone Initiative (NVROI): Insights to understanding air pollution in complex terrain

Abstract

The Nevada Rural Ozone Initiative (NVROI) was established to better understand O3 concentrations in the Western United States (US). The major working hypothesis for development of the sampling network was that the sources of O3 to Nevada are regional and global. Within the framework of this overarching hypothesis, we specifically address two conceptual meteorological hypotheses: (1) The high elevation, complex terrain, and deep convective mixing that characterize Nevada, make this state ideally located to intercept polluted parcels of air transported into the US from the free troposphere; and (2) site specific terrain features will influence O3 concentrations observed at surface sites. Here, the impact of complex terrain and site location on observations are discussed. Data collected in Nevada at 6 sites (1385 to 2082 m above sea level (asl)) are compared with that collected at high elevation sites in Yosemite National Park and the White Mountains, California. Average daily maximum 1-hour concentrations of O3 during the first year of the NVROI ranged from 58 to 69 ppbv (spring), 53 to 62 ppbv (summer), 44 to 49 ppbv (fall), and 37 to 45 ppbv (winter). These were similar to those measured at 3 sites in Yosemite National Park (2022 to 3031 m asl), and at 4 sites in the White Mountains (1237 to 4342 m asl) (58 to 67 ppbv (summer) and 47 to 58 ppbv (fall)). Results show, that in complex terrain, collection of data should occur at high and low elevation sites to capture surface impacts, and site location with respect to topography should be considered. Additionally, concentrations measured are above the threshold reported for causing a reduction in growth and visible injury for plants (40 ppbv), and sustained exposure at high elevation locations in the Western USA may be detrimental for ecosystems.

Entrainment of stratospheric air and Asian pollution by the convective boundary layer in the southwestern U.S.

“…Concentrations at the LVV sites were very similar to those measured at Angel Peak during the afternoons but diverged during the night when shallow nocturnal boundary layers created by strong radiative cooling in the high desert decoupled the surface layer from the afternoon mixed layer. Similar patterns were observed at several of the NVROI sites [ Gustin et al ., ]. This was followed by rapid depletion of surface O 3 at these sites through deposition and titration, the latter playing larger roles at Joe Neal and Jerome Mack where there are more local sources of NO x (cf.…”

“…The NAAQS has gradually approached these background concentrations with time, and the recent reduction [U.S. Environmental Protection Agency, 2015] in the NAAQS from 75 to 70 parts per billion by volume (ppbv) adds urgency to the need for better understanding of the processes that control ground-level background ozone in the United States [Cooper et al, 2015]. This is particularly true for the southwestern U.S. and Intermountain West, where surface ozone peaks in late May and early June with daily maximum 8 h average (MDA8) concentrations that approach and sometimes exceed the revised standard in both urban and rural areas [Fine et al, 2015a;Fiore et al, 2014;Gustin et al, 2015;Langford et al, 2012;Langford et al, 2015b;Lefohn et al, 2014;Lefohn et al, 2012;Lin et al, 2012a;Lin et al, 2012b;Zhang et al, 2014]. and Viezee, 1981;Viezee et al, 1983] and, to a lesser degree, transport of ozone formed from anthropogenic precursors emitted in Asia [Jacob et al, 1999;Jaffe et al, 1999].…”

“…Our analysis suggests that the exceptionally deep (3-5 km asl) convective boundary layers that form above the Mojave desert in late spring [Fine et al, 2015a;Fine et al, 2015b;Seidel et al, 2012] were able to entrain air from stratospheric intrusions and associated Asian pollution that had passed intact over the shallower boundary layers of the San Joaquin Valley and the Sierra Nevada Mountains. We hypothesize that this process, which can impact major population centers like Las Vegas and Phoenix in addition to remote areas like the Death Valley, Grand Canyon, and Great Basin National Parks [Gustin et al, 2015], is a major contributor to the springtime ozone maximum in the southwestern U.S. and will complicate implementation of the lower 70 ppbv NAAQS in this region. stratospheric intrusions above Angel Peak coincided with several days of high surface ozone in and around Clark County and exceedances of the 75 ppbv NAAQS on 21 and 25 May.…”

Air Quality at Devils Postpile National Monument, Sierra Nevada Mountains, California, USA

“…All of these California-based high elevation locations show average summertime diurnal maxima in the range of approximately 50-60 ppb during daytime hours, with great site-to-site variability at night. The same general trends are also present in the data from the Nevada Rural Ozone Initiative (NVROI), which sampled six different sites at elevations ranging from 1385 to 2082 masl Gustin et al, 2015). The observation of a relatively constant/recurring average diurnal maximum across different sites and different years suggests that these rural locations are all influenced primarily by background concentrations of O 3 , with smaller contributions (on average) from local photochemical production or regional transport of intact air masses.…”

“…These investigations indicate that exceedances of the new 70 ppb NAAQS for O 3 are likely to occur more frequently because of increasing background O 3 (Gratz et al, 2014;Fine et al, 2015;Gustin et al, 2015). The importance of elevated background O 3 is especially pronounced during the spring, when both longrange transport (i.e., from outside North America) and stratospheric intrusions exhibit seasonal maxima (Cooper et al, 2011;Lin et al, 2012;Lin et al, 2015).…”